Production of titanium nitride



' #5477416 CHAMBER INVENTOR PETER I? ALEXA/V058 ATTORNEYS MECHA/WML MIX/N6 DE VICE P. P. ALEXANDER PRODUCTION OF TITANIUM NITRIDE Filed March 2, 1945 77W 4- Ca 0 Feb. s, 1949.

NITROGEN (AMMONIA) Patented Feb. 8, 1949 OFFICE PRODUCTION TITANIUM NITRIDE Peter P. Alexander, Beverly, Mass, assignor to;

Metal Hydrides Incorporated, Beverly, Mass., at corporationof Massachusetts Application March 2, 1945, Serial No. 580,688 7 This invention relates to titanium nitride and has for its object certain improvements in the method of producing titanium nitride.

It is customary in the production of titanium nitride to pass a stream of nitrogen or ammonia over a heated body of the titanium metal to be nitrided. If the titanium is in a form of substantial size, there is a tendency for the nitride ingaction to take place only at the surface. If the surface of the'titanium is not clean and clear of an oxide coating, for example, the nitriding action is greatly inhibited. To overcome these difficulties, the titanium is used in finely divided form and special precautions are taken to assure clean metallic surfaces on the individual particles. There is a marked tendency for fusion of the non-nitrided titanium particles to take place due to excessive exothermic heat generated by the titanium that is nitrided. As fusion of the non-nitrided titanium sets in, the particles agglomerate and then merge into a larger body, which may take in substantially all of the particles of titanium and titanium nitride. In addition, the titanium tends to fuse against the container in which the nitriding reaction is conducted. The net result is to obtain an imperfectly nitrided titanium product, contaminated with the metal of the container, in the form of a fused mass that cannot be removed from thecontainer without great difficulty. This method is inefficient and costly.

As a result of my investigations, I have discovered a relatively efi'icient and inexpensive method of producing titanium nitride of high quality. The necessary finely divided metallic titanium is obtained by the reduction of a'finely divided titanium compound in a suitable reaction chamber. The freshly reduced titanium particles are then converted to the desired titanium nitride in the same reaction chamber as part of the same operation.

In accordance with the invention, a charge of finely divided titanium compound, such as the oxide, chloride and iodide, and a suitable reducing agent, such as a reducing metal or reducing metal compound, is confined in a reaction zone. The charge is heated to a temperature sufficiently high to reduce the titanium compound. Nitrogen or ammonia is then admitted to the reaction zonev 5 Claims. (c1. 23-491) f air and moisture, the reaction'zone and charge are advantageously heated to a suitable elevated temperature. While the reduction operation may be conducted while the reaction zone and charge are under vacuum, the springing of a leak in the reaction chamber is apt to result promptly in the seepage of outside air and moisture into the chamber. This undesirable result-may be avoided by breaking the vacuum by admitting an inert gas, such as helium or argon, to the reaction zone. Enough inert gas is admitted to place the reaction zone and charge under substantial positive pressure, for example 5 pounds. If the reduction chamber should. spring a leak, inert gas would tend to seep from the chamber to the outside atmosphere and thus prevent air and moisture from seeping into the reaction chamber. To maintain this balance, the positive pressure of the inert gas in the chamber is maintained by the admission of further amountsof inert gas, if necessary.

Among the reducing agents that maybe employed to reduce the titanium compounds are the alkaline earth metalscalcium, barium and strontium-and magnesium. 'Amongthe reducing metal compounds that may be employed are the reducing metal hydrides, such as the hydride of the alkaline earth metals, and particularly calcium hydride becauseof its availability. The charge is preferably formed of intimately admixed and finely divided particles of titanium compound and reducing metal compound, such as a reducing metal hydride, for example, calcium hydride, or reducing metal, the reducing agent being used in slight excess; in other words, slightly more than that theoretically-required to effect reduction of all of the titanium compound present in the charge.

The charge in the reaction zone is heated to a temperature sufliciently high to initiate and complete the reduction operation. Since the titanium compound and the reducing agent are present in the charge in finely divided form, the resulting reduced titanium is also in'the form of finely divided particles distributed throughout the resulting mass of finely divided compound of the reduced agent.

To illustrate a practice of the invention, the

starting material advantageously is finely divided titanium dioxide pigment, which as made today is of high purity. Since a reducing metal such as metallic calcium, for example, cannot be converted readily to a finely divided'form' for intimate admixture with the finely divided titanium dioxide, and since an excessiveamount ofjexoparticles of calcium oxide.

thus eliminating or minimizing the problem of fusing the resulting reaction mass.

In the case of a charge of intimately admixed titanium dioxide pigment and finely divided calcium hydride particles, the reductionoperation results in finely divided titanium metal particles distributed throughout a mass of refractory calcium oxide particles. The reaction may be indicated as follows:

It appears that the hydrogen gas released by dissociation of the calcium hydride may act as a purifying agent, by reacting with and therefore removing traces of surface oxide from the titanium metal-particles. Such a side reaction would form water, which would at once be converted tosteam at the temperature employed.

The removal of the surface oxide from the particles of titanium is a cleansing action and therefore better adapts the titanium for the nitriding i reaction.

If desired, the hydrogen gas releasedby the main reaction as Well as the steam released by the side reaction, together with inert gas, if present, areremoved from :the'reaction zone by placing it under vacuum, If desired, inert gas is then fed to the reaction; zone-to placeit under positive pressure.

Nitrogen of ammonia is admitted to the reaction zone while the charge is still hot to convert the reduced titanium metal' to its nitride. The freshly reduced titanium particles present substantially clean metallic surfaces, and are therefore in optimum condition for nitriding. The reduced titanium particles are converted by the nitrogen or ammonia to titanium nitride.

The alternative reactions may. be indicated as follows: 7

2Ti+N2+2TiN 2Ti+2NH3+2TiN+3H2 There is a considerable evolution of heat as the nitriding reaction takes place so that the temperature of the charge may be quickly elevated. Since the reduced titanium particles are widely distributed and held more or less separately in the mass of refractory compound of the reducing metal, there is little opportunity for the non-nitrided titanium particles to fuse together. The reduced titanium particles are widely distributed and held more or less separately in the resulting mass of refractory calcium oxide. Even though there is a considerable evolution of heat as the nitriding reaction proceeds and the temperature of the charge rises appreciably, the fusingztog'ether of thetitanium particles is' largely-inhibited because they are separated from one. another by 1 the protective Due'to the presence. of a slight excess of the reducing metal, traces "of oxygen present in the charge and'reactionzone combine with the reducing metalzand'therefore protect the reduced Furthermore, an exces- 4 titanium as well asthe newly formed titanium nitride. A certain amount of the nitrogen may also combine with any reducing metal left over. For this reason, it is preferable to use the reducing metal only in slight excess, particularly when it is desired to obtain a high purity metal nitride product, although this is not so important in cases where the nitride of the reducing metal may be readily leached from the main metal nitride. Thus, calcium nitride is leached from titanium nitride in the leaching step described below.

In a presently preferred practice, the inert gas is retained in the reaction c h amber as the nitrogen or ammonia is admitted. Regulated amounts of nitrogen or ammonia may be thus admitted to regulate the rate of nitriding and hence the rate of generation of exothermic heat. Objectionable overheating of the charge may thus be prevented.

To facilitate the reduction operation or the nitriding operation, or both, the charge may be mixed as. it takes place. A special mixing device may be used for the purpose, or the charge may be suitably tumbled. N

If the nitriding agent is ammonia, it dissociates at the high temperatures employed, and the desired nitriding action then takes place. It is thought by some that nascent nitrogen resulting from the dissociation of the'ammonia has a greater afiinity for the titanium metal than normal nitrogen. My investigations lead me to believe that when ammonia is dissociated at high temperatures, it results in pure nitrogen and pure hydrogen; the pure hydrogenreacts with and removes traces of surface oxides on the titanium metal particles to be nitrided, thus providing clean metallic surfaces; and the pure nitrogen then reacts with the titanium metal particles to form titanium nitride. In other words. the hydrogen of the ammonia appears to function as a scavaging agentwhich purifies the titaniurn metal particles for reaction with the nitrogen.

The resulting reaction mass is permitted to cool, say to room temperature, in its atmosphere of nitrogen, after which the titanium nitride is suitably separated fromthe compound of the reducing. agent. This may be done, for example, by crushing the reaction mass, if necessary, and leaching out the compound of the reducing agent with a suitable solvent, such as a dilute solution of a suitable acid, for example acetic acid. The wet titanium nitride is then dried and stored for use.

These and other features of the invention will be better understood by referring to the accompanying drawing, taken in conjunction with the following description, which illustrates diagrammatically an apparatus usable in a practice of the invention:

The apparatus shown comprises a pot retort l8 suitably suspended within a furnace H having a heating chamber l2 with a conduit 53 near the bottom through which to introduce heating gases into the chamber, and'a flue opening M near the top thereof through which to remove spent gases from the chamber. The retort is advantageously madeof heat-resistant steel. It

tially of the retort between of spaced supports l8 extending circumferenthe flange and the top of the furnace.

The cover is provided with a pipe I9 having a valve I9 for the controlled passage of nitrogen or ammonia, a pipe 29- having a valve connecting the other pipe for the controlled passage of inert gas, such as helium or argon, into the retort; a charging conduit 2| to supply titanium compound, such as titanium dioxidepigment, and a reducing agent, such as calcium hydride, to the interior of the retort. As shown, the conduit is fitted with three spaced valves 22, 23 and 24, to provide an evacuating passageway 25 and a trap 26 of sufficient size to receive a substan, tial charge, for example, of intimately admixed titanium dioxide pigment and calcium hydride 21. A pipe 28 with a valve 29 connects the evacuating passageway with a source of vacuum, not shown.

A mechanical mixing device 30 fits within the interior of the retort. It is provided with a vertical shaft 3|, the upper end of which extends through the cover and is suitably held in position by a bearing 32 and a collar 33 secured to the shaft. The upper end of the shaft is also equipped with a pulley 34 connectible with a source of power, not shown. The lower part of the shaft is fitted with a plurality of lateral supports 35 to which is attached a helical metallic ribbon 36 adapted to follow thecontour of the inside of the retort so as to raise chargematerials confined therein along the inside surface of the retort, and then to roll them toward the cen ter, thereby intimately mixing the charge materials.

A discharge conduit 49 connects with the bottom of the retort and extends through the bottom of the furnace. A valve 4| is provided in the conduit below the furnace bottom for the re moval of residues from the retort.

In accordance with a practice of the invention, cover I5 is appropriately bolted to flange I6 to assure a sealed joint. Heating gases are passed through conduit l3 into heating chamber I2, so that the bottom of retort I8 is heated. Spent heating gases escape through flue opening I4. Valves 24, 23 and 29 are opened and valves I9, 20', 22 and ll are closed until the air, moisture and other gaseous products are evacuated from the interior of the retort; This may be accomplished by connecting conduit 28 with a'vacuum pump, not shown. To hasten the evacuation step, the temperature of the interior of the retort is suitably raised. The charge may also be mixed, by rotating device 39, as it is placed under vacuum to help remove air and moisture.

On completion of the evacuation step, valves 24 and 29 are closed, valve 22 is opened and charge 2'! of titanium dioxide and calcium, in slight excess, is passed into conduit H and dropped into trap 25; after which valve 22 is closed and valve 29 is opened to evacuate air and moisture from trap 26 and charge 2?. 23 and 29 are then closed and valve 25 is opened so that the charge is dropped into the retort.

Additional heating gases are passed into heating chamber I2 to raise the temperature of the charge within retort I-U to a temperature at which the calcium hydride is dissociated into metallic calcium and hydrogen gas and at which the reduction of the titanium dioxide by the calcium may proceed satisfactorily. Device 39 is rotated to mix the charge intimately. While the temperature may be checked in various ways,

Valves it is practical to employ'ahollow Shamani which is placed a suitablethermocouple. In a presently preferred practice, valve2ll' is opened, after charge?! is dropped intoretort I 0 and valve 24 is closed; toadmitinert gas, such as helium-or argon, or both, in amount'sufiicient to place and maintain the reaction zone under substantial positive pressure, for example .5 pounds. The charge is then heated and mixed before and during the reduction reaction.

As the titanium dioxideiparticles are reduced, the resulting metallictitanium particles are distributed throughout the resulting mass of cal cium oxide particles. The calcium oxide particles tend to separate the reduced titanium particles. In other words, the titanium particles have little opportunity to contact one another because they are suspended in a mass of calcium oxide particles. c

As noted above, the temperature at which reduction takes place is also high enough to dissociate the calcium hydride into metallic calcium and hydrogen gas. Some of the hydrogen gas probably unites with surface oxide on the reduced titanium metal particles to form water whichis promptly converted to steam. To insure a more complete conversion of the available titanium metalto high purity titanium nitride, the retort is advantageouslyevacuated to remove the hydrogen gas, steam and inert gas. This maybe accomplished by openingvalves 24, 23 and 29. They are preferably then closed and valve 20 again opened to admit enough inert gas to place the reaction zone under substantial positive pressure, after which valve 20' is closed.

Valve I9 isthen opened to admit controlled amounts of nitrogen or ammonia to the interior of the retort. Unless the temperature of the reaction mass within the retort is insuflicient to initiate the nitriding reaction between the freshly reduced titaniumparticles and the nitrogen or ammonia, introduction of further heating gases into the heating chamber is terminated. As pointed out above, the reaction between titanium and nitrogen is an exothermic one and, unless special precautions are taken, objectionable overheating tends to take place, thereby causing fusion of the titaniumparticles and impairment of the .nitriding'reaction. To prevent this unde sirable result, the. titanium particles are nitrided' as they are distributed in and among the calcium oxide particles. In a preferred practice, the inert gas is retained in the retort so that the nitrogen or ammonia ma be admixed and diluted therewith. If the inert gas in the retort is maintained at an optimum pressure, the amount of exothermic heatreleased'by the nitriding reaction may be controlled byregulating the rate at which nitrogen or ammonia is admitted to the retort. This may be done automatically by feeding additional amounts of nitrogen or ammonia to the retort when the pressure within the retort falls below a predetermined level, but not in amount to exceed a predetermined level, the range being such as to limit the supply of nitrogen or ammonia to an amount which is insufficient to raise the temperature to an objectionable extent. A suflicient amount of nitrogen or ammonia is thus admitted to effect complete conversion of the reduced titanium to titanium nitride.

The retort and its contents are then permitted to cool. Valves I9 and 29 are closed and cover I 5 is removed. The titanium nitride and calcium oxide are then removed from the retort. This may be done by opening valve III and rotating the anc ors mixing. device, when the reaction mass is discharged through conduit ill. The reaction mass is crushed, if desired, and leached with dilute acid, such as hydrochloric or acetic acid. The acid dissolves out the calcium oxide, as well as any calcium nitride that may have formed, leaving the titanium nitride, which is dried and stored for use.

It will be clear to those skilled in this art that the above example is only byway of illustration; that the practice of the invention readily lends itself to a number of useful. modifications; and that the invention offers a relatively efficient and inexpensive method of producing titanium nitride.

Reference may be made to my three (3) copending applications filed March 2, 1945, which also relate to metal nitrides: Serial Nos. 580,687; 580,689; and 580,690; the claims of which are directed broadly to the production of metal nitrides and specifically to the nitrides of zirconiu and tantalum, respectively.

I claim:

1. A method for producing titanium nitride which comprises confining in a reaction zone a charge of an intimate mixture in finely divided form of titanium oxide and a metal containing reducing. agent capable of forming a refractory metal oxide by reduction of the titanium oxide, heating the charge sufliciently high. to reduce the titanium oxide by reaction with said reducing agent and form a reaction mass in which finely divided particlesof titanium metal are distributed and held in a mass of particles of refractory oxide, then admittingto the reaction zone a gas selected from the group consisting of nitrogen and ammonia, and converting the titanium metal to titanium nitride while distributed and held in the protective refractory oxide.

2. A method for producing titanium nitride which comprises confining in a reaction zone a charge of an intimate mixture in finely divided form of titanium oxide and a metal hydride, heating .the charge sufficiently high to reduce the titanium oxide by reaction with said metal hy'- dride and form a reaction mass in which finely divided particles of titanium metal are distributed and held in amass of particles of refractory oxide, then admitting to the reaction zone a gas selected from the group consisting'of nitrogen and ammonia, ,and converting the titanium metal to titanium nitride while distributed and held in the protective refractory oxide;

3. A method for producing titanium nitride which comprises confining in a reaction zone a charge of an intimate mixture in finely divided form of titanium oxide and a reducing metal, heating the charge sufficiently high to reduce the titanium oxide by reaction with said reducing metal and form a reaction mass in which finely divided particles of titanium metal are distributed and held in a mass .of particles of refractory oxide, then admitting to the reaction zone a gas selected from the group consisting of nitrogen and ammonia, and converting the titanium metal to titanium nitride while distributed and held in the protective refractory oxide.

4. A method for producing titanium nitride which comprises confining in a reaction zone a charge of an intimate mixture in finely divided form. of titanium oxide and calcium hydride, heating the charge sufficiently high to reduce the titanium oxide by reaction with said calcium hydride and form a reaction mass in which finely divided particles of titanium metal are distributed and held in a mass of particles of refractory oxide, then admitting to the reaction zone a gas selected from the group consisting of nitrogen and ammonia, and converting the titanium metal to titanium nitride while distributed and held in the protective refractory oxide.

5. A method for producing titanium nitride which comprises confining in a reaction zone a charge of an intimate mixture in finely divided form of titanium oxide and magnesium, heating the charge sufiiciently high to reduce the titanium oxide by reaction with said magnesium and form a reaction mass in which finely divided particles of titanium metal are distributed and held in a mass of particles of refractory oxide, then admitting to the reaction zone a gas selected from the group consisting of nitrogen and ammonia, and converting the titanium metal to titanium nitride while distributed and held in the protective refractory oxide.

PETER P. ALEXANDER.

REFERENCES {JITED The following references are of record in the file of this patent:

UNITED STATES PATEIITS Number Name Date 996,032 Serpek June 20, 1911 1,077,712 Heyder Nov. 4, 1913 1,088,909 Kuzel Mar. 3, 1914 1,180,840 Giulini Apr. 25, 1916 1,274,797 Shoeld Aug. 6, 1918 1,343,441 Farup June 15, 1920 1,366,720 DeLoisy Jan. 25, 1921 1,533,505 Lubowski Apr. 14, 1925 1,803,720 Miner May 5, 1931 OTHER REFERENCES Chemical Abstracts, vol. 35, 1941, page 4712 Certificate of Correction Patent No. 2,461,018. February 8, 1949. PETER P. ALEXANDER It is hereby certified that error appears in the printed specification of the above numbered patent requiring correction as follows:

Column 3, line 42, for the words Nitrogen of read Nitrogen or;

and that the said Letters Patent should be read with this correction therein that the same may conform to the record of the case in the Patent Ofiice.

Signed and sealed this 14th day of June, A. D. 1949.

THOMAS F. MURPHY,

Assistant Oommissiomr of Patents. 

